Inability to cultivate Mycobacterium leprae (M. leprae) in vitro has been a major bottleneck in leprosy research. Today, the leprosy bacillus remains the only bacterium causing disease in man that has not been cultured in vitro and until this is achieved, all studies on leprosy will remain at a serious disadvantage compared with other human bacterial infections. Since the discovery of the organism, many attempts have been made to cultivate it in vitro. Although many successes have been claimed, growth has not yet been confirmed. Ultrasensitive methods have been developed to quantitate ATP and DNA content of M. leprae and also to determine its metabolic capability using 3H-thymidine. The signal for cell division is coded for in the terminal segment of the replicating bacterial chromosome. Thus, the overall objective of this proposal is to obtain chromosomal replication in suspensions of M. leprae. Ultrasensitive methods will be used to meausre the changing content of the DNA/ATP ratio in suspensions of leprosy bacilli as it relates to the nutritional environments. Complete replication of the chromosome followed by the initiation of cell division are the first steps to be achieved in order to successfully grow M. leprae. Various factors will be scrutinized to find ways to keep the membranes of M. leprae intact when transferred from in vivo to in vitro environment and thus overcome the initial physiologic shock. Later several metabolic blocks and growth factors will be explored to find the optimal combination of nutrients for bacteria to survive and multiply. Finally, the characters of in vitro-grown M. leprae will be compared with the characters of their in vivo-grown counterparts in order to ascertain the authenticity of the in vitro-grown organism. The successful in vitro cultivation of M. leprae would provide a universally available and simplified techenique for drug sensitivity testing, for screening new anti-leprosy drugs and for large-scale supplies of M. leprae readily available to the production of vaccines and a wide range of skin-test antigens and anti-leprosy antisera.